1. Field of the Invention
The present invention relates to a transfer apparatus for transferring an image onto a transfer medium by means of a belt member and an image forming apparatus based thereon.
2. Description of the Related Art
In a conventional image forming apparatus such as a laser printer adopting the electrophotography, the process speed is low in many cases, requiring a photosensitive member having only a small diameter. It was therefore the conventional practice to accomplish separation by use of the rigidity of the transfer medium, and provide a stripping charger such as a transfer corona in the downstream of the transfer apparatus as auxiliary means, thereby neutralizing the transfer medium to an extent not causing disturbance of the image. However, along with the recent progress of digitization of image forming apparatuses of copying machines, a high process speed is making it difficult to effect separation by use of rigidity of the transfer medium. There is therefore proposed a method known as the transfer belt process using a belt for the transfer member, and electrostatically absorbing the transfer medium to the belt, thereby conveying and transferring the image.
On the other hand, an electrophotographic full-color image forming apparatus having an intermediate transfer member in addition to an image carrier such as a photosensitive drum is known. This is based on a process comprising the step of repeating a plurality of times a so-called primary transfer which transfers a toner image formed on an image carrier once onto an intermediate transfer member to laminate toner images of a plurality of colors onto the intermediate transfer member, and then secondary-transfers these toner images of the plurality of colors in a lump onto a transfer medium such as a sheet of paper.
While applicable types of intermediate transfer members include the intermediate roller type, apart from the intermediate belt type, the intermediate belt type is more excellent than the intermediate roller type in a higher degree of freedom of arrangement and a higher separability (permitting curvature separation) of the transfer medium after secondary transfer.
The corona transfer process has conventionally be used commonly for transfer purposes because of the advantages including easier construction of the apparatus. However, to cope with environmental problems such as generation of ozone, the general tendency is now directed toward adoption of the contact transfer process.
However, when driving a transfer medium carrying/conveying belt serving as a belt member by bringing the same into contact with a photosensitive drum, or by bringing an intermediate transfer belt serving as a belt member into contact with the photosensitive drum in an image forming apparatus as described above, impossibility to achieve a uniform circumferential speed for both the drum and the belt results in a difference in circumferential speed, and this causes a problem of wearing or damage to the drum and the belt. Wear of, or damage to these components in turn causes problems of defective image or reduction of service life.
To solve these problems, Japanese Patent Laid-Open No. 10-221964 discloses a technique of preventing wearing damage caused by the contact of the transfer medium carrying/conveying belt with the photosensitive drum by separating the contact transfer means except during image formation, and providing an auxiliary member for making up for a decrease in the tension of the loosening belt by separating the contact charging means. With the technique disclosed by Japanese Patent Laid-Open No. 10-221964, however, it is necessary to provide new auxiliary means for maintaining the belt tension. This leads to a more complicated configuration and to the need for ensuring a space for the auxiliary means, resulting in a cost increase and scaling-up of the apparatus. When problems such as those mentioned above are encountered between the intermediate transfer belt and the photosensitive member including frictions or other damage between the intermediate transfer belt and the photosensitive member, it is impossible to cope with these problems.
According to Japanese Patent Laid-Open Nos. 5-53410 and 7-140805, color shift can be prevented by providing a support structure permitting simultaneous swinging of a pulley opposite to the photosensitive member and a tension pulley, making the photosensitive member and the intermediate transfer member separable from each other, and adopting a configuration not causing a decrease in the intermediate transfer member belt tension. However, because of the necessity to simultaneously move two rollers for keeping a constant belt tension, the structure becomes more complicated, leading to an increase in cost.
An object of the present invention is to ensure stable belt conveyance by moving the transfer member between a transfer position and a non-transfer position while maintaining the tension of the belt member.
The present invention provides a transfer apparatus comprising a belt member, a plurality of belt supporting members which drive the belt member while stretching the belt member with a prescribed tension, a transfer member, which is one of the plurality of belt supporting members, and transfers an image formed on an image carrier toward the belt member, and transfer member moving means making it possible only for the transfer member movable between a transfer position where the image is transferred and a non-transfer position where the image is not transferred while maintaining the prescribed tension.
The present invention provides also an image forming apparatus comprising image forming means which forms an image on an image carrier, a belt member, a plurality of belt supporting member which drive the belt member while stretching the same with a prescribed tension, a transfer member which is one of the plurality of belt supporting members, and transfers an image formed on the image carrier toward the belt member, and transfer member moving means which makes it possible only for the transfer member to a transfer position where the image is transferred and to a non-transfer position where the image is not transferred, while maintaining the prescribed tension.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings).